Broadening the spatial frequency pass band of a thermoplastic layer

ABSTRACT

A layer of thermally deformable plastic used, for example, in a holographic recording medium has an undulated surface whereby the layer has an uneven thickness. Because of the uneven thickness, the layer has a composite spatial frequency pass band greater than the spatial frequency pass band of a layer of uniform thickness.

This is a division, of application Ser. No. 653,288, filed Jan. 29,1976.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to holography and more particularly to aholographic recording medium comprised of a thermally deformableplastic.

2. Description of the Prior Art

In a typical hologram of an image, each portion of the image is recordedat a multiplicity of locations on a recording medium. Therefore, thehologram is inherently a highly redundant recording. Because of theinherent redundancy, when, for example, portions of the hologram aredisfigured or discarded, an undiscarded, clean portion can provide aholographic reconstruction of the entire image. The inherent redundancyis a desirable feature of the hologram which is practially unknown inother types of recording.

The image may be holographically recorded on a medium, such as a tape,which typically includes a layer of a photoconductor intermediate to alayer of an electrically conductive material and a layer ofthermoplastic. Accordingly, the thermoplastic has a surface abutting thephotoconductor and a free surface.

The thermoplastic is comprised of a dielectric which becomes soft inresponse to an application of heat. Materials such as thermoplastic areknown in the art as thermally deformable plastics.

To holographically record an image on the medium, a uniform voltage ismaintained between the conductive layer and the free surface. Inresponse to the photoconductor being provided with an interferencepattern of light representative of the image, an electrical charge onthe thermoplastic varies spatially in a pattern of a diffraction grating(comprising the hologram of the image).

The pattern of the charge establishes a corresponding pattern ofelectrostatic force between opposed surfaces of the thermoplastic. Inresponse to an application of heat, the free surface is softened wherebythe diffraction grating is formed therein by the electrostatic force.

When the diffraction grating is used to provide a holographicreconstruction of the image, portions of the image are usuallyinaccurately reconstructed. The inaccurate reconstruction is caused by acorrespondence of portions of the image to spatial frequencies which arenot within a spatial frequency pass band of the thermoplastic wherebythe portions are not efficiently recorded. Typically, the pass band isapproximately equal to 400 cycles per millimeter. Additionally, the passband has a center frequency known in the art as a frost frequency. It iswell-known that the frost frequency is inversely proportional to thethickness of the thermoplastic.

Usually, the 400 cycles per millimeter pass band is on the order of arange of spatial frequencies comprising the hologram of the image(referred to as the image range hereinafter). Therefore, it is desirableto have the frost frequency substantially equal to the center frequencyof the image range. Since the frost frequency is inversely proportionalto the thickness of the thermoplastic, the thickness is of criticalimportance when the thermoplastic is fabricated; a slight error in thefabrication which causes an error in the thickness results in the frontfrequency deviating from the center of the image range thereby causingan inefficient recording. The critical importance of the thicknesscauses the recording medium to be expensive and unreliable. Heretofore,the inherent redundancy of a hologram has not been used to broaden thespatial frequency pass band of a layer of thermoplastic.

SUMMARY OF THE INVENTION

According to the present invention, in a holographic recording medium, alayer of thermally deformable plastic has an uneven thickness wherebythe layer has a composite pass band greater than the pass band of alayer of a thermally deformable plastic having a uniform thickness.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side elevation of a segment of tape in accordance with apreferred embodiment of the present invention;

FIG. 2 is a graphic showing of the spatial frequency pass band of athermoplastic layer of FIG. 1; and

FIG. 3 is a side elevation of a segment of tape in accordance with analternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred embodiment, a thermoplastic layer has an uneventhickness thereby causing the thermoplastic to have a multiplicity ofpass bands. The uneven thickness is an essential feature of the presentinvention.

The multiplicity of pass bands form a composite pass band whereby animage, holographically recorded on the thermoplastic, is accuratelyreconstructed when the hologram of the image is comprised of spatialfrequencies within the composite pass band.

Referring to FIG. 1, in the preferred embodiment of the presentinvention, a segment of a holographic recording tape includes an activeportion 10 having a thickness of approximately two microns. The activeportion 10 is comprised of a photoconductor layer 12 intermediate to ametallic layer 14 and a thermoplastic layer 16 whereby the thermoplastic16 has a free surface 18. Because of being only two microns thick, theactive portion 10 is not self-supporting and therefore, is carried by amylar substrate 20 which is a supporting structure.

The surface 18 is undulated whereby the thickness of the thermoplastic16 is uneven. Preferably, thermoplastic 16 is constructed to provide thesurface 18 with undulations having a spatial frequency much lower thanspatial frequencies included in a diffraction grating comprising ahologram. Undulations having the much lower frequency do not diffractlight and therefore, are not part of the diffraction grating.

It is well-known that spatial frequencies of the diffraction grating areon the order of 1,000 cycles per millimeter. Accordingly, theundulations should have a frequency less than 100 cycles per millimeter.

The surface 18 has locations 21, 22, 23 where the thermoplastic 16 has a0.6 microns (0.0006 millimeters) maximum thickness, a 0.4 micronsminimum thickness, and a 0.5 microns intermediate thickness,respectively. In this embodiment, the distance between the locations 21,22 (one-half cycle) is one-half of a millimeter whereby the spatialfrequency of the undulation is one cycle per millimeter. It should beunderstood that on the surface 18 a hologram may be recorded asdescribed hereinbefore or in any other suitable manner.

Referring to FIG. 2, illustration (a), a spatial frequency response ofthe thermoplastic 16 at the location 22 is represented by a curve 24which is centered about an abscissa 26 representative of an upper frostfrequency. In this embodiment, the abscissa 26 represents a frequency of1,250 cycles per millimeter.

Correspondingly, a spatial frequency response at the location 21 isrepresented by a curve 28 centered about an abscissa 30 representativeof a lower frost frequency. In this embodiment, the abscissa 30represents a frequency of 733 cycles per millimeter.

In a similar manner, a spatial frequency response at the location 23 isrepresented by a curve 32 centered about an abscissa 34 representativeof an intermediate frost frequency. In this embodiment, the abscissa 34represents a frequency of 1,000 cycles per millimeter.

Referring to FIG. 2, illustration (b), since the surface 18 isundulated, the thermoplastic 16 has a multiplicity of frost frequenciesin a range from 733 cycles per millimeter to 1,250 cycles permillimeter. Therefore, the thermoplastic 16 has a multiplicity of passbands which form a composite pass band represented by a curve 36.

Referring to FIG. 3, in an alternative embodiment of the presentinvention, a segment 39 of a holographic recording tape includes anactive portion 38 comprised of a photoplastic layer 40. The photoplastic40 is a thermally deformable plastic which is photoconductive.Accordingly, the photoplastic 40 has combined properties of thethermoplastic 16 and the photoconductor 12 described hereinbefore.

The photoplastic 40 is disposed upon the metallic layer 14 whereby theactive portion 38 is carried by the substrate 20. Additionally, thephotoplastic 40 has an undulated surface 42 similar to the surface 18referred to hereinbefore. Therefore, because of the surface 42, thephotoplastic 40 has a multiplicity of pass bands which form a compositepass band. The hologram is recorded on the segment 39 in a mannersimilar to recording a hologram of an image on thermoplastic.

In other embodiments, the uneven thickness may be provided by athermally deformable plastic layer having the shape of a wedge.

What is claimed is:
 1. In the method of recording a hologram of an imagein a medium wherein an electrically conductive layer has first andsecond surfaces connected to one surface of a photoconductor layer and asurface of a substrate, respectively, the steps of:providing athermoplastic layer of substantially uneven thickness that causes saidthermoplastic layer to have a multiplicity of frost frequencies;disposing said thermoplastic layer upon the other surface of saidphotoconductor layer, whereby said photoconductor layer is between saidconductive layer and said thermoplastic layer and said thermoplasticlayer has a surface abutting said photoconductor layer and a freesurface; maintaining a uniform voltage between said conductive layer andsaid free surface, thereby providing a charged assembly; providing aninterference pattern of light representative of said image to saidphotoconductor layer in said charged assembly; and applying heat to saidcharged assembly.
 2. The method of claim 1 wherein said step ofproviding said thermoplastic layer includes the step of providing saidthermoplastic layer with a multiplicity of frost frequencies within arange of 733 cycles per millimeter to 1,250 cycles per millimeter.
 3. Inthe method of recording a hologram of an image in a medium wherein onesurface of an electrically conductive layer has one surface connected toa substrate, the steps of:providing a photoplastic layer ofsubstantially uneven thickness that causes said photoplastic layer tohave a multiplicity of frost frequencies; disposing said photoplasticlayer upon the other surface of said conductive layer; maintaining auniform voltage between said conductive layer and said free surface,thereby providing a charged assembly; providing an interference patternof light representative of said image to said photoplastic layer in saidcharged assembly; and applying heat to said charged assembly.
 4. Themethod of claim 3 wherein said step of providing said photoplastic layerincludes the step of providing said photoplastic layer with amultiplicity of frost frequencies within a range from 733 cycles permillimeter to 1,250 cycles per millimeter.